The present invention relates to a method for forming a semiconductor device having a crystalline silicon semiconductor film such as a polycrystalline silicon film, a single crystalline silicon film and a microcrystalline silicon film. The crystalline silicon film formed by the present invention is used in various semiconductor devices.
A thin film transistor (TFT) using a thin film semiconductor is well known and constructed by forming a thin film semiconductor, particularly a silicon semiconductor film. The TFT is used in various integrated circuits and attracted as, in particular, a switching element provided with each pixel and as a driver element formed in a peripheral circuit portion in an active matrix type liquid crystal display device.
As a silicon film used in a TFT, it is convenient to use an amorphous silicon film. However, there is a problem that an electrical characteristic of the amorphous silicon film is very low than that of a single crystalline semiconductor used in a semiconductor integrated circuit. Therefore, the amorphous silicon film is used in only limited use such as a switching element in an active matrix circuit. To improve a characteristic of a TFT, a crystalline silicon film may be used. The crystalline silicon film other than a single crystalline silicon is called a polycrystalline silicon (poly-silicon) film, a microcrystalline silicon film or the like. To obtain such crystalline silicon film, an amorphous silicon film may be formed and then crystallized by heating (thermal annealing). This is called a solid phase growth method because an amorphous state is changed into a crystalline state while maintaining a solid state.
Since a solid phase growth for silicon needs a heating temperature of 600° C. or higher and 10 hours or longer, it is difficult to use an inexpensive glass substrate as a substrate. Since, for example, a Corning 7059 glass used in an active matrix type liquid crystal display device has a glass warp point (glass transition temperature) of 593° C., when a large size substrate is used, a thermal annealing with 600° C. or higher produces a problem.
To contrast this, according to research of the inventors, the following has confirmed. That is, an extremely small quantity of elements such as nickel, palladium, lead or the like is deposited on a surface of an amorphous silicon film and then heated, so that crystallization can be performed at 550° C. for about 4 hours.
To introduce an extremely small quantity of elements (a catalytic element which promotes crystallization) as described above, a film containing the catalytic element or a compound thereof may be deposited by sputtering. However, if a semiconductor includes a large quantity of elements as described above, reliability and electrical stability of a device using such semiconductor is deteriorated. When a film is formed by sputtering, it is difficult to accurately adjust a quantity, that is, a thickness of the film. Also, it is further difficult to form a film having a uniform thickness on a substrate. Therefore, variations in a characteristic of a semiconductor device to be obtained produce.
Also, When a film is formed by sputtering, since an amorphous silicon film is damaged largely by shock in sputtering, a characteristic of a semiconductor device is not always preferred.
There is a method for forming a film by, for example, a spin coating, in stead of sputtering. However, it is difficult to obtain a film having a uniform thickness by the spin coating. For example, in a rectangular substrate used in a liquid crystal display device, a solution concentrates easily on corners of the substrate, so that a film thickness is nonuniform. Also, when a film containing a catalytic element compound is formed by drying a solvent, a film thickness is nonuniform by nonuniformity of drying and generation of crystal nucleus, so that it causes variations of semiconductor devices.